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The US Army uses a light tactical High-Mobility Multi-Purpose Wheeled Vehicle (HMMWV) which, due to the amount of armor added, requires air conditioning to keep its occupants comfortable. The current system uses R134a in a dual evaporator, remote-mounted condenser, engine-driven compressor system. This vehicle has been adapted to use an environmentally friendly refrigerant (carbon dioxide) to provide performance, efficiency, comfort and logistical benefits to the Army. The unusual thermal heat management issues and the fact that the vehicle is required to operate under extreme ambient conditions have made the project extremely challenging. This paper is a continuation of work presented at the SAE Alternate Refrigerants Symposium held in Phoenix last June [1].

Phase separation circuiting have been proved in the past to effectively improve the performance of mobile air conditioning (MAC) condensers. In the vertical second header of the condenser, liquid separates from vapor mainly due to gravity, leaving vapor-rich flow with higher heat transfer coefficient to go into the upper passes. The condenser effectiveness is improved in this way. However, separation is usually not perfect, expressed through the separation efficiency (ηl and ηv). This paper presents the numerical study of phase separation phenomena in the second header. The Euler-Euler method of Computational Fluid Dynamics (CFD) is used. Simulations are conducted for two-phase refrigerant R-134a for MAC application. Inlet mass flow rate is simulated at values of 16 g∙s-1, 20 g∙s-1, and 30 g∙s-1 for 21 inlet microchannel tubes, which is the same 1st-pass tube number as of a real separation condenser. Corresponding mass fluxes are 166 kg∙m-2∙s-1, 207 kg∙m-2∙s-1, and 311 kg∙m-2∙s-1.